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Missing bottom paint

Discussion in 'Ask All Sailors' started by Bill22113, Jan 12, 2018. Add this thread to a FAQ

  1. Bill22113


    Joined Aug 19, 2005
    11 posts, 0 likes
    Cal 33 MkII
    US Clinton ,CT
    Last summer my Cal33 lost patches of copper bottom paint around all bronze through hulls. Had cleaned all bonding connections before launch and used aluminum prop shaft anodes instead of zinc (which never lasted a season). Aluminums less than half gone by haulout. Why the copper bottom paint loss?

  2. rgranger


    Joined Jan 19, 2010
    4,865 posts, 855 likes
    Hunter 26
    US Smith Mountain Lake
    You switched from zinc. My first guess is electrolysis. Aluminum has an electrochemical oxidation potential of -1.706 V and the copper in your paint has a reduction potential of 0.3402 V


    2(Al ---> Al+3 + 3e-) +1.706
    3(Cu+2 + 2e- --> Cu) +0.3402
    2Al + 3Cu+2 ---> 2Al+3 + Cu =2.05V

    You have a 2+ volt electrochemical difference between the copper in your paint and the aluminum sacrificial anode.

    A zinc anode has an oxidation potential of 0.762 .... almost a volt lower potential.

    If you think of voltage as being analogous to pressure..... then the higher the voltage difference, the more pressure two dissimilar metals have to react. It probably was happening with your zinc just at a slower pace. Let me ask you this.... did you have growth problems in the affected area? If not.. I would not worry about it.

  3. Bill22113


    Joined Aug 19, 2005
    11 posts, 0 likes
    Cal 33 MkII
    US Clinton ,CT
    Thanks rgranger. Perhaps I'll only use one Al anode this year. Still looking for cause of rapid zinc anode consumption in previous years but that's another topic.

  4. capta


    Joined Jun 4, 2009
    2,415 posts, 592 likes
    Pearson 530
    na Admiralty Bay, Bequia SVG
    Zincs can be affected by a multitude of things that have nothing to do with YOUR boat. An old engine block close to your slip, a steel boat or two on the docks, a leaking shore power connection on one of the dock pedestals. The list is endless.
    When I started sailing, there were still boats with copper sheathing on their hulls and if a steel boat came into that marina (or vice versa), everybody's zincs would go ballistic.
    It was my understanding that the alloy zincs were for fresh water only. Perhaps that thinking has changed? If you really want to know what is going on with your boat, a surveyor or marine electrician will have a meter for measuring your 'leakage', if any. Call one and get your boat tested.

  5. Gunni


    Joined Mar 16, 2010
    5,070 posts, 957 likes
    Beneteau 411 Oceanis
    US Annapolis
    Suspicion: Your newly optimized bonding system turned your boat into a more effective partner in marina stray current activity. The aluminum anodes are supposed to last longer which until you get this sorted out is a good thing for your prop and exposed drive.

  6. thinwater


    Joined Mar 26, 2011
    1,886 posts, 321 likes
    Corsair F-24 MK I
    US Deale, MD
    Google "paint burn back."

    fstbttms likes this.
  7. fstbttms


    Joined Feb 26, 2011
    1,048 posts, 81 likes
    Achilles SD-130
    US Alameda, CA
    Yes, it's called "burn back" and is caused by improper priming of underwater metals.

  8. dlochner


    Joined Jan 11, 2014
    2,088 posts, 728 likes
    Sabre 362
    113 US Fair Haven, NY
    Capta, if the "alloy zincs" you refer to are the Aluminum and Magnesium anodes, then rest the thinking has changed. Aluminum can be used in fresh, brackish or salt water. Magnesium only in fresh water, zinc only in saltwater. has good information on this.

  9. cagreen75


    Joined Nov 6, 2017
    14 posts, 2 likes
    Catalina Catalina 30
    US Stratford
    I had an issue that may be related to yours. When the boat was hauled in 2016 there were no zincs left. The previous year the zincs were worn but still there. I installed a brand new zinc on the feathering prop and one the shaft prior to launching in the spring. I had not changed slips but I did get a new neighbor. As I researched this I found that the likely cause was electrolysis. I checked all the grounds on my boat and could not find any issues so I came to the conclusion that the problem was with either a boat near me with a grounding issue or the problem was the shore power connection. Rather than try to get the marina to test the shore power circuits I decided to check all the boats that were near mine during the season. When I final found my new neighbor’s boat his zincs were also gone. With that I concluded that one of us had a problem. I decided that I would double the shaft zinc to two and install the normal one on the feathering prop. And just to be safe I also installed a galvanic isolator. This year when the boat hauled the zincs were more than 60% left. I just happened to be at the marina when the hauled my neighbor’s boat and there were again no zincs left. My conclusion to all of this is there may be a problem with his boat that is affecting boats around it. I say this with a bit uncertainty as he did move to a slip further away on the other side of the dock However when I looked at the two boats that were next to his last season their zincs were gone or mostly gone. Although the galvanic isolator my be saving my zincs and consequently my expensive prop I really don’t know, but I do know that if there is a shore power problem it will protect my boat. I think it is good insurance. Your problem may just be the way the fitting was prepared before painting. There is a primer that is specifically designed to prime underwater metal parts that also my help you.

  10. dlochner


    Joined Jan 11, 2014
    2,088 posts, 728 likes
    Sabre 362
    113 US Fair Haven, NY
    If the problem is always around or near a particular boat, then the issue is the boat. The shore power problem is a function of a safe shore based AC circuit. The solution is install a galvanic isolator on your boat to protect your boat from those other boats that have issues. Shore side AC is wired such that the ground (green) and the neutral (white) wires are connected at the service panel. This allows current to pass from one boat to another via the ground as all boats are connected via the ground wire even when the boat's AC circuit breakers are turned off.

    A properly wire boat AC system has a main circuit breaker that switches both the hot (black) and neutral wires. Shore based circuit breakers only break the hot wire, leaving the neutral and ground permanently connected.

    Hope that is clear.

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